Change the color depth

The lag in Tetris is not going to be related to your colour depth, and changing colour depth is not going to make it any faster (and may just make it slower, particularly if you succeed in moving to a palette based colour mode).

Presuming that it's a Flash-based app in the browser, it'll have everything to do with Flash support.

There are several issues here:

• Adobe no longer support Flash on linux, so you can't get a recent version. Depending on your Ubuntu version you may be able to still get Flash 11.2, from the flashplugin-installer package in multiverse - otherwise follow instructions here for using the "pepper" support in Chrome.

• If you don't use Adobe's own Flash plugin, you'll be using one of the open source alternatives, which are likely to have very poor, slow performance.